In recent years, new types of lasers using a photonic crystal have been developed. A photonic crystal consists of a dielectric body material in which an artificial periodic structure is created. Usually, the periodic structure is created by providing the body material with a periodic arrangement of areas whose refractive index differs from that of the body material (this area is hereinafter called the “modified refractive index area”). The periodic structure causes a Bragg diffraction within the crystal and creates an energy band gap for the energy of light. There are two types of photonic crystal lasers: one utilizes a band-gap effect to make a point-like defect function as a resonator, and the other utilizes a standing wave at a band edge where the group velocity of light becomes zero. Each of these devices causes a laser oscillation by amplifying light of a predetermined wavelength.
Patent Document 1 discloses a two-dimensional photonic crystal laser in which a two-dimensional photonic crystal is created in the vicinity of an active layer containing a luminescent material. The two-dimensional photonic crystal includes a plate-shaped member in which circular holes (i.e. modified refractive index areas) are periodically arranged (e.g. in a triangular or square lattice pattern) so as to provide the crystal with a two-dimensional, periodic distribution of refractive index. Its period is adjusted so that it equals the wavelength of light to be generated within the active layer by an injection of carriers from an electrode. As a result, a two-dimensional standing wave is produced within the two-dimensional photonic crystal, whereby the light is strengthened to produce a laser oscillation. The laser light is diffracted by the circular holes to a direction perpendicular to the active layer and two-dimensional photonic crystal, and emitted in this direction.
In the case where circular holes are used as described in Patent Document 1, the electric field of light in the two-dimensional photonic crystal encircles the (gravity) center of each circular hole and is anti-symmetrical with respect to the gravity center. The anti-symmetry of the electric field cancels the electric field at every hole due to interference (or destructive interference). If the two-dimensional photonic crystal has an infinite extent, the electric field will be completely cancelled due to such a destructive interference, so that the laser light cannot be extracted perpendicularly to the two-dimensional photonic crystal. Actually, the extent of the two-dimensional photonic crystal is finite. Therefore, the electric field cannot be completely cancelled, so that the laser light will be extracted. However, the strength of the laser light will not be sufficient due to the influence of the destructive interference.
Patent Document 2 discloses a two-dimensional photonic crystal laser which utilizes modified refractive index areas having a characteristic shape in order to prevent the destructive interference. In each modified refractive index area, no part of the modified refractive index area lies on a first half-line extending from the gravity center thereof in a direction within a plane of the two-dimensional photonic crystal, while at least a part of the modified refractive index area lies on a second half-line extending from the gravity center in a direction opposite to the first half-line. As an example of the modified refractive index area having such a shape, Patent Document 2 discloses a V-shaped modified refractive index area 91A (FIG. 1A) and a cluster modified refractive index area 91B (FIG. 1B) having an equilateral-triangular arrangement of three identically-shaped modified refractive index areas as a unit. Both in the V-shaped modified refractive index area 91A and in the cluster modified refractive index area 91B, no modified refractive index area lies on the first half-line 921 extending from the gravity center G in one direction, whereas a modified refractive index area lies on the second half-line 922 extending in the direction opposite to the first half-line 921.
Using a modified refractive index area having such a shape results in a difference in the refractive index between the first half-line side and the second half-line side. This suppresses the destructive interference, so that the laser light can be emitted with a greater strength than in the case of using a circular modified refractive index area.